Process and plant for the purification of raw maleic anhydride recovered from gaseous reaction mixtures

Abstract: In the purification of maleic anhydride obtained with an oxidation process, by means of absorption with an organic solvent and fractionation of the resulting liquid phase and purification of the crude maleic anhydride thus separated, the above purification being accomplished through two serially placed fractionating columns, the accumulation of polymerisable acrylic acid overhead the first fractionating column of the purification section, and of water as well as of maleic acid overhead the second fractionating column of the purification section, is avoided by carrying the overhead vapours of said first fractionating column to an absorption tower, preferably operating with the same absorption organic solvent used to absorb the crude maleic anhydride from the reaction mixture, and by carrying the overhead vapours of said fractionating column of said purification section to two condensers placed in series so that the gaseous fraction, coming out of the first condenser, is fed to said second condenser.

Description: The present invention refers to a process for the purification of maleic anhydride and, more particularly, to a process for the purification of the crude maleic anhydride resulting from the recovery by means of absorption with an organic solvent of the same anhydride from gaseous mixtures obtained from butane and/or benzene oxidation. The maleic anhydride is produced by an oxidation reaction in gaseous phase and is obtained as a diluted and raw, gaseous form. A first recovery method consists in the water absorption of the maleic anhydride. which reacts with water to form maleic acid. After the water separation by distillation and the dehydration of the maleic acid, maleic anhydride is recovered. This recovery type involves, however, a high energy consumption, product loss owing to conversion of maleic acid to fumaric acid, high amounts of pollutants in the effluents and frequent plant stops for washings. Alternatively, since some time the recovery of maleic anhydride by absorption in an organic solvent has been proposed. To this process, proposed in the U.S. Patent 2,942,005 describing the absorption of maleic anhydride and phtahlic anhydride from reaction mixtures, several operating improvements have been brought. In the US Patent 3891680, the gaseous reaction mixture (from which maleic anhydride is to be recovered) is contacted with an organic liquid phase. A similar process is disclosed in the U.S. Patent 4118403, in which the maleic anhydride is recovered by absorption in an organic liquid to which phthalic anhydride is added. Finally, the U.S. Patent 5969687 discloses the maleic anhydride recovery by means of an organic solvent and, in particular, includes a stripping step of the enriched solvent with a gas of low humidity contents to remove water and volatile substances. In all the procedures based on the use of an organic absorption solvent, the enriched solvent is passed through a separator under vacuum, in which the raw maleic anhydride is condensed overhead and the depleted solvent leaves from the bottom for being recycled to the absorption phase. As already mentioned, this fractionating step is common to all solvent recovery processes and leads to produce raw maleic anhydride of a 97-99% purity. The main residual impurities consist of: (i) a small amount of solvent (ii) a certain amount of organic by-products formed in the oxidation reactor and absorbed together with the maleic anhydride in the absorber, and (iii) water, either free or combined with maleic anhydride. Typical examples of light organic impurities are acrylic acid and traces of carbonyl compounds. If the solvent is an ester of a light alcohol (e.g. dibutylphthalate) and the separation between maleic anhydride and solvent is conducted under severe conditions, the alcohol that comes out from the decomposition of the organic solvent might also be present as an impurity in the raw maleic anhydride, typically in the monoester form. Even though the raw maleic anhydride purification can be carried out both in batch and continuous processes, the batch process has some major disadvantages, as for example, the discontinuous steam demand and the necessity of bringing considerable amounts of maleic anhydride to boil, with a consequent, increased risk of exothermal reactions of the maleic anhydride. The continuous process appears to be preferable and, according to a classical scheme entails a first fractionating column, within which the light substances are removed and a second column from the bottom of which the solvent containing a small quantity of maleic anhydride is drawn, that solvent being sent to recycling, whilst the recovered maleic anhydride is drawn in liquid form from the overhead section. However, also the continuous process faces problems and drawbacks not yet solved. The first of these relates to the drawing of overhead products from the first column where the light substances are separated, including acrylic acid. The fraction drawn from overhead, and that is sent to a condenser for recycling in liquid phase to the column head, contains acrylic acid in a high concentration, which tends to polymerize and thus to foul the condenser and the column uppermost part in a not negligeable extent. The second problem connected to the traditional process is represented by water and light or volatile substances present in the second purification column, where the maleic anhydride is recovered in the upper part of the column itself. This water might originate from atmospheric humidity infiltrating in the vacuum operating system, or from the decomposition of maleic acid present in the raw maleic anhydride. Water may also be possibly produced by losses from condensers or boilers, due to leaking tube-to-tubesheet joints. Since such water is being absorbed into the maleic anhydride condensed overhead the second purification column and refluxed overhead the column itself, an unwanted concentration increase of water, that is, of maleic acid, in the maleic anhydride obtained in this column, occurs. Purpose of the present invention is to substantially eliminate such problems and drawbacks. This purpose is achieved by the process of the present invention for the purification of raw maleic anhydride coming from a recovery unit, of the type in which the maleic anhydride present in the gaseous reaction mixture is absorbed by means of an organic solvent into an absorber, and the enriched organic solvent is sent to a vacuum separation column, from which raw maleic anhydride is drawn overhead, whereas the depleted solvent is recycled to said vacuum separation column, the maleic anhydride being then sent to a purification section, the process being characterized in that: the top section of the purification column is operated under stripping conditions, whereby the vapours drawn overhead are fed to an absorber which is countercurrently fed with an organic solvent, preferably the same solvent used for the absorption of the raw maleic anhydride from the reaction mixture, the solvent phase coming out from this absorber and containing all the maleic anhydride of the overhead section of the purification column, and most of the acrylic acid being recycled to said first absorption column for absorbing with an organic solvent the raw maleic anhydride from the reaction mixture. According to a preferred embodiment of the invention, the vapours drawn from overhead the first purification column, before being fed to the absorber, are sent to the inlet of a condenser which can be the same condenser of the vacuum separation column, the raw maleic anhydride being its overhead product, where most of maleic anhydride is condensed. In turn, the plant according to the invention for the purification of crude maleic anhydride of the type comprising a first column for the absorption with an organic solvent of raw maleic anhydride from the reaction mixture that comes from the oxidation reactor, vacuum separation column fed with the enriched solvent phase coming from said first absorption column, and a purification section fed with the overhead product of said vacuum separation column, said purification section including a fractionating column, said plant being characterized in that it comprises: a vacuum absorption tower fed with the overhead gaseous fraction outgoing from said first fractionating column, possibly after partial condensation of said overhead gaseous fraction. The distinctive aspects and advantages of the present invention will be clearer from the following detailed description, related to the enclosed drawings that schematically show a preferred embodiment of the plant for the carrying out of the process of the present invention. More specifically, figs. 1,2 and 3 show the plant section of the maleic anhydride recovery and of the first part of the purification section, according to possible changes. By reference first to the fig. 3, reference 10 represents the separation column by means of absorption with an organic solvent of the maleic anhydride that comes from the oxidation reactor. Reference 12 precisely indicates the feed of gaseous mixture coming from the oxidation reactor, whereas reference 14 stands for the feed of the absorption organic solvent. The exhausted gases are discharged through outlet 16. From the absorber 10, the solvent phase enriched with maleic anhydride is fed, as per reference 18, to a fractionating column 20, from the bottom of which the depleted solvent phase 22 is drawn and recycled to the absorber 10, whilst reference 24 indicates altogether and in general the bottom reboiler of the column 20. The gaseous phase outgoing from head of the tower 20 is sent, as indicated by reference 26, to a condenser 28, from which a gaseous phase, consisting of uncondensable gases, is obtained and sent for the disposal as shown by reference 30, whilst the condensate (line 32) feeds the first fractionating column 34 of the purification section. From the bottom of the column 34 a liquid phase 36 is obtained, consisting of maleic anhydride which is sent to the second purification column (as per fig. 4), while the gaseous or overhead phase, drawn as reference 38 indicates, contains the previously mentioned impurities and, particularly, the acrylic acid that, by polymerisation, solidifies and causes severe scale and fouling problems of the column head 34. In accordance with the present invention, the above head fraction, instead of being subjected to condensation with recycling of the liquid phase overhead the column 34, is directly sent to a vacuum absorber 40 which, in the embodiment of fig. 3, is fed by the same organic solvent for the absorption of the raw maleic anhydride from the gaseous reaction mixture in the absorption tower 10, that is, the depleted organic solvent recycled from the bottom of the fractionating column 20. The bottom fraction deriving from this absorber is sent through line 42 to the first absorbing tower 10 paralleles to the recycle of depleted solvent 14, whilst the overhead fraction, that includes almost mainly uncondensable and volatile products, is sent to the disposal as indicated by reference 44, wherein preferably the gaseous fraction resulting from the condenser 28 also is fed. It is noteworthy that in this manner the problem related to the presence of no negligeable traces of acrylic acid in the raw maleic anhydride is substantially avoided, without however undergoing a loss of maleic anhydride, as much the bottom or liquid fraction from the vacuum absorber 40 passes to the primary absorber 10, where the gaseous reaction mixture subjected to the absorption, operates as a stripping gas to remove the acrylic acid. In the embodiment form illustrated in fig. 1, the gaseous or head phase 38 of column 34, before being sent to the vacuum absorption column 40, is partially condensed in a condenser 60, the condensate of which is recycled (through line 62) to the feed 32 of the first fractionating column 34, while the gaseous phase through line 64 is sent to the vacuum absorption column 40. Fig. 2 shows a change of this embodiment as, instead of a separate condenser 60, there is used the same condenser 28 provide for the condensation of the overhead product 26 of column 20. In figures 1-3 enclosed herewith, the preferred embodiment forms of the two parts of the purification section are shown, being understood that it is possible and envisageable to design the purification section with the fractionating column 34 made as shown in figs. 1,2 or 3. With an industrial plant built up with the variarious of the two purification columns, realized as shown in the drawings, industrial tests of the process in accordance with the invention have been carried out. The tests have been conducted by checking the concentrations of maleic anhydride. acrylic acid and water at the positions that are typical of the plant and of the related process. The data, referred to 1000 kg/h of treated raw maleic anhydride are reported in the following table Maleic anhydride Acrylic Acid Water (as free water and maleic acid) Raw maleic anhydride 1000 2,34 4,78 purification column (34)-- overhead 200 2,12 4,40 purification column (34)-- bottom 800 0,22 0,34 air leakages entering the column (46) - - 0,37 gaseous phase from condenser (52) 25 0,19 0,51 liquid from condenser (58) 23 0,12 0,27 exhaust gas from condenser (58) 2 0,07 0,24 final product (48) 750 0,03 0,20 From the data of the previous table it is clear that in the overhead fraction of the first purification column, about 20% of the maleic anhydride to be purified is present together with a large part of the acrylic acid and water. In the subsequent steps, more than 99% of this maleic anhydride is recovered (particularly in the vacuum absorption tower 40). In addition, the uncondensable vapours from the outlet of the first condenser 52 contain most of the water and of the acrylic acid which are fed to the second purification column 46. These vapours are condensed in the second condenser and recycled upstream so as to extract the light components. With the process and plant of the invention, the purified maleic anhydride has an acrylic acid content definitely lower than 0.01% and a maleic acid content definitely lower than 0.05%, whereas with the traditional processes and plants, the acrylic acid content may be about 0.01% or higher, and that of maleic acid may be about 0.05% or more. Table 2 indicates the typical values for a traditional purification section, operated without using the process of the invention, wherein from overhead the first column a liquid and a gaseous phase are extracted and the second column is equipped with only one condenser for the overhead product. Also for this table the reported data refer to a 1000 kg/h feed of raw maleic anhydride. Maleic anhydride Acrylic acid Water (as free water and maleic acid) Raw maleic anhydride 1000 2,34 4,78 purification column (34) -- overhead (lig. + vap.) 20 2,10 4,35 purification column (34) -- bottom 980 0,24 0,39 leakages in (46) - - 0,37 exhaust gas from the only condenser of column (46) 2,8 0,10 0,24 final product 950 0,14 0,52

Claims: A process for the purification of raw maleic anhydride coming from a recovery unit, of the type wherein the gaseous reaction mixture (12) is subjected to separation by absorption in an absorber (10) of the raw maleic anhydride by means of an organic solvent (42,14) and the enriched organic solvent (18) is sent to a vacuum separation column (20), from overhead of which raw maleic anhydride is drawn (26), whilst the depleted solvent (22) is recycled to said separation column (10), the raw maleic anhydride being subsequently sent to a purification section that consists of a fractionating column (34) characterized in that the top part of the said purification column (34) is operated under stripping conditions, whereby the overhead vapours (38) are drawn and fed to an absorber (40) which is in turn fed countercurrently with an organic solvent, the solvent phase (42) from the outlet of this absorber, containing all the maleic anhydride present in the overhead fraction of the said purication column (34) and a large portion of the acrylic acid, being recycled to said first column (10) for the absorption with an organic solvent of the crude maleic anhydride from the reaction mixture. A process according to claim 1, characterized in that the vapours (38) drawn from overhead said purification column (34), before being recycled to said first absorption column (10),are sent to a condenser (60 or 28), operating under conditions for the condensation of the raw maleic anhydride. A process according to claim 2, characterized in that said condenser (60) is placed in series with said separation column (34) and downstream of same. A process according to claim 1 and/or 2, characterized in that said organic solvent, used in said vacuum absorption column (40) fed with the overhead product (38) of said purification column (34), is the same solvent used for the absorption of the raw maleic anhydride from the reaction mixture in the absorber (10). A plant for the purification of raw maleic anhydride, of the type which comprises a first column (10) for the absorption with an organic solvent of raw maleic anhydride from the reaction mixture (12) coming out from the oxidation reactor, a vacuum separation column (20) fed with the enriched solvent phase (18), coming out of said first absorption column (10), and a purification section fed with the overhead phase (26) of said separation column (20), said purification section including a fractionating column (34), characterized by comprising a vacuum absorption tower (40) fed with the overhead gaseous fraction (38) coming out of said fractionating column (34), the liquid phase, consisting of solvent and maleic anhydride as well as of acrylic acid that comes from the bottom of said vacuum absorber (40), being recycled to said tower (10) for the absorption of raw maleic anhydride from said gaseous reaction mixture. A plant according to claim 5, characterized in that said overhead gaseous fraction (38) coming out from said first fractionating column (34) of said purification section is sent to a condenser (60) for an at least partial condensation thereof. A plant according to claim 5, characterized in that said overhead gaseous fraction (38) is sent to a condenser (28) placed at the outlet of the overhead gaseous product (26) of said second under-vacuum separation column (20) for an at least partial condensation thereof.

Inventors: CONNI SERGIO, RUGGIERI ROBERTO, CONNI, SERGIO, RUGGIERI, ROBERTO
Applicants: SISAS SPA, SISAS SOCIETA'ITALIANA SERIE ACETICA E SINTETICA SPA
Application year: 1994
Main classification: C07C57
IPC codes: C07C51